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What to do when it is too warm

I am pushing this one out a bit earlier than I had expected. Typically, we get a few hot days in June - nearly always they seem to correspond with Southeastern Wisconsin Trout Unlimited's Driftless work day and fishing gathering which would have been this weekend or last (June 5th or 12th, as I write this). And typically, we settle into what I will call a "mid-summer pattern" some time around the beginning of July. This year, the heat has been unprecedented, a word we throw around too much. However, as I write this, we are not 11 full days into June and by this afternoon, we will have set 8 record high temperatures for the month. This will break the previous record for the number of record high temperatures for an entire month. Unprecedented.

A bit of perspective...

First, some good news. We have been pretty fortunate over the last number of decades that we have not, knock on wood, suffered a prolonged drought. Those of us old enough to remember 1988-9 remember just how bad things can get. A bit of perspective - May and June of 1988 saw just 2.9 inches of rainfall, the lowest ever recorded in nearly 150 years of record keeping in Wisconsin. Parts of southwest Wisconsin were over 15 inched below their average annual precipitation. And, of course, the drought was not just in Wisconsin nor was it just a one year blip. That is what is a bit odd about this year - we have been in a 40 year period with elevated average precipitation - granted with few short-term droughts thrown in.

Source: from Chris Kurcharik, UW-Madison Department of Agronomy.

An interesting bit of perspective from the image above - it is dry and 2021 is an outlier but compared to the decades from 1960 to about 2000, it is not that out of place. Now, droughts do not happen in just a few months. Add in a winter with relatively little precipitation and that has an affect. As does the record heat we have been experiencing. Am I concerned? Certainly but it is early and things can change quickly. As anyone living or visiting the Driftless knows, we have dealt with more flood events than we have droughts. Things can change - and fast - but yeah, we could use some rain.

The good news is that our spring fed streams are generally in really good shape. Decades of mostly above average - or with climate change, a new average - precipitation, our aquifers are in great shape. The figure above is from the Wisconsin DNR report, "Evaluation of trout population trends and fisheries management in the West Fork Kickapoo River Watershed" and shows how the amount of water in our streams (Discharge, the top two panels of the figure) and precipitation have been on the incline for decades. One year of dry weather - and really so far, a dry spring and moderately dry winter - is not going to change all of that at once.

What about the trout?

The trout will generally be fine - they have survived for millions of years - but not during much of that time have they had to deal with us humans. We are - without question - a "game changer". I think we owe it to them to be smart and make good decisions about fishing. I have not fished much lately because, well, who the Hell wants to be outside right now? But if I did feel the need to go fishing, I would, 1) choose when and where I fish for trout wisely, or 2) chase something other than trout.

Headwater stream with watercress
Look for watercress as an indicator of springs - or just carry a thermometer...

If I just had to go chase trout, I would:

  1. fish early in the day,

  2. fish stream reaches that I know stay cold,

  3. streams that are shaded,

  4. fish a heavier rod and heavier tippet,

  5. and more than anything I would carry a thermometer with me.

A little biology / geology. Springs tend to come out of the ground at approximately the average annual air temperature of the location (this is an approximation - there are other factors at play) and springs are extremely consistent in temperature. This means that in the winter, the water from the springs is relatively warm and in the summer, it keeps the stream cold. I have placed a temperature logger in a spring and recorded it for two years in the past. The temperature never deviated more than 0.2*C - within the error of the temperature logger. For almost all of Wisconsin, spring flow is what allows trout to survive in our streams. A common, and I was a bit surprised to learn a few years back, a non-native indicator of springs is watercress (Nasturtium officinale). In most streams, the density of springs is higher and they have a greater influence as you move upstream.


Carry a thermometer! All the other things in the list help - coldwater inputs and things that help keep the water cool like shading, cooler overnight temperatures, orientation of the stream and valley, and avoiding causing more stress to the fish less like fishing heavier tippet, not keeping them out of the water for long - or at all. All of these things help but none of them are as important to fish survival as not fishing, wading, or otherwise stressing fish when the water temperatures at which trout are not able to consume more calories and oxygen than they expend.

These recommendations are not only for the fish but for you. As the water temperatures hit the mid to upper 60's (Fahrenheit; around 19.5*C), fish get to a point where they are less mobile and willing to feed so fishing is almost always going to suck. The amount of oxygen that water can hold decreases as water temperature increases and trout require much more oxygen than most fishes. Eventually as water temperature increases, fish require more energy for metabolism (requiring more oxygen) than they can obtain. Because of how trout have evolved in cold, well oxygenated waters; the range of temperatures in which trout are able to survive is quite low compared to most fishes. And likewise, there are few species of fishes that can grow as trout do in cold water. For more details on this topic, I wrote about fish bioenergetics a few months ago.

Inexpensive digital thermometer
An inexpensive digital thermometer is accurate, easy to read, and easily carried.

A stream thermometer is a cheap insurance policy for the fish and for your fishing. I just bought another thermometer - this one with a bottle opener. My choice of thermometers are the "instant read" meat thermometers that are digital, have a large backlit display, and have a display that rotates. It cost me less than $20, it is accurate within at least 1*C (2*F), and can be calibrated in a glass of ice water. And the bottle opener is a nice bonus!

Two branches some together
A north-south (L) and west-east (R) branch come together and show really different effect of shading.

There are a lot of factors that affect how warm - or cool - a stream reach will be. Typically streams are coldest in their headwaters and warm as we move downstream. However, I know more than a couple of places where the very headwaters are not the coolest reach of the stream and it is not until spring flow increases that the stream cools. For some streams, one really large spring or tributary totally changes the thermal regime. Orientation of the streams can have a huge effect too. I was out last week on small headwater stream where two spring-fed branches came together and noticed how different the two reaches were (see above image). The one that ran north to south had almost no weeds or periphyton (algae, diatoms, bacteria, and other organisms attached to rocks) and the branch that flowed west to east was full of periphyton. I have had some better luck in the summer finding streams that flow north/south rather than east/west, particularly on "high sun" days.

Stream temperatures can vary greatly from reach to reach and even within the course of a day. A conversation just this morning (not today, but as I write this...) with PJ Smith, whom has spent a lot more time fishing recently than I have, said that one stream rose by about 10 degrees (F) over the course of a day. Another was 54 degrees Fahrenheit and stayed that temperature all day. There is so much variation based on spring flow and your proximity to springs, orientation of the stream, heat sinks that warm some streams up (flat, shallow areas with dark bottoms and ponds are the worst culprits), and other factors. It is difficult to take all of these things into account - so carry a thermometer.

Branch out, try something different

Trout streams are warm? Give something else a try. There are so many other great fishes to chase other than trout. Live in a place without a ton of access to streams that stay cold? Give other fish a try on the fly rod. Smallmouth Bass are the perfect fly rod fish, particularly in rivers. I have the Mississippi River not far away and can chase a ton of different species. Probably my favorite on the river is to fish bass with topwaters, particularly frogs, out of my kayak. A totally overlooked option are carp on the fly. Talk about a challenge! They are smart, hard to get a hook into, and once hooked, put up a hell of a fight.

Fisheries Management and Warm Streams

Historically, most states have assumed that people are not idiots. Problematic, I know. I kid (sort of...). But many states - including Wisconsin - have had to take more drastic measures. You may be familiar with Montana's "hoot owl" restrictions that limit fishing from 2 PM until midnight (12 AM). The idea is that their mostly freestone streams increase in temperature during the day and cool nights cool the streams allowing anglers to fish early in the day. Hoot owl restrictions have been enacted in Montana, Washington, California, and other states. These restrictions have typically been placed on larger freestone rivers that are largely feed by winter snowfall and rely upon cool nights and rainfall in the summer to cool them. Hoot owl restrictions in Western states tend to be applied to specific streams when temperature criteria have been met.

In 1990 as the effects of the drought were being felt, Wisconsin closed streams to fishing in the southwest part of the state and had others only open to catch and release. The effects of the drought lasted several years and there are some streams that had their Brook Trout populations extirpated (local extinction) that have still not recovered. This was about the time I started trout fishing in Wisconsin and information on the closures is hard to find online. I would love feedback from those that remember the 1988-1990 drought better than I do.

To be clear, I do not think we are at the point, right now, that any of these more drastic measures are needed. For now, we should rely upon people being smart and not doing things to unnecessarily cause damage to trout populations. Carry a thermometer and use it - knowing that warm streams are most likely to occur later in the day. Do your part, be smart, and give the trout a fighting chance. Maybe do a little rain dance...

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